1. Field of the Invention
The device described herein is a new design for a sampler head or sample cutter for the extraction of representative sample increments from a material stream conveyed on a belt conveyor. The sampler is of a type generally called a cross-belt sampler, but is also known as a go-belt sampler or a hammer sampler.
The new design corrects the technical faults that are present in the known designs. The novel design principle for the sample cutter geometry can in fact be used to show that all the known designs for cross-belt sampler cutters, with one exception, take a sample that is not representative of the material on the belt.
2. Prior Art
Sampling of bulk materials conveyed on a conveyor belt is a critical operation in the transfer of bulk commodities between buyer and seller or within industrial processing operations. It is usual to sample the commodity as it is loaded onto transport (road, rail or ship) and to sample it as it is off-loaded at the buyer's site or at the boundaries of the processing operation. The sample that is analysed to estimate the quality or value of the shipment must be representative of the entire shipment. A sample is said to be representative of the material sampled when the expected assay of the sample is equal to the true mean assay of the lot of material being sampled. Modern statistical sampling theory provides a means of determining how many sample increments and the total mass of material that must be extracted from the flow of material as it is loaded or off-loaded in order to ensure that the expected value of the absolute difference between the properties of the accumulated sample and the true properties of the entire shipment is limited to an acceptable magnitude. The increments extracted from the stream are combined into the representative sample and that sample may be further sub-sampled by other devices to arrive at the final mass of material submitted for physical and chemical analysis. If all the sampling equipment used in taking and processing the sample increments and the final sample is correctly designed, any difference between the properties of the accumulated sample and the true properties of the entire shipment will be randomly distributed with some variance and a statistical expected value of zero. When the statistical expected value of this difference is zero, the sampling equipment is said to be unbiased. Lack of bias in commercial sampling of commodities is usually a contractual requirement and it is essentially mandatory that sampling equipment be unbiased.
The demonstration that a sampling device is unbiased is accomplished in two stages. First, an analysis of its geometry and motion through the stream of material to be sampled must show that, at an arbitrary position in the stream, the time interval during which material can enter the sampler is a constant when the speed of the sampling device through the stream remains constant. If this criterion is not met, and there is segregation of the particles in the process stream with respect to particle size or particle composition that arises due vibration or the manner of loading the material onto to the belt, the sample increments will be biased. The extent of the bias will depend on the extent of segregation and any failings of the design. If a sampling device can be shown to meet this first criterion of lack of bias, a practical test can be carried out to verify that it is indeed unbiased under circumstances of practical operation. It is important to verify that all the particles that are supposed to be collected into an increment are collected without any overflow of the sampling device and that particles that are not supposed to be collected are not collected; it can happen that even though the sampling device has a suitable geometry and motion, it imparts momentum by friction or impact to particles that should not be collected and these particles are collected into the increment. An optimal design for a sampling device will meet the first criterion on its geometry and its motion and will also have design features that minimise the momentum transfer to particles that are not part of the increment to be collected. A sampling device that operates at constant speed and meets the first criterion is said to be mechanically correct.
In its most simple form, the cross-belt sampler is a flat rectangular surface equipped with two parallel side plates to delimit the sample (see FIG. 1) and swings through the load of solids on the conveyor belt in a plane normal to the direction of travel of the belt. Such a sampler geometry is disclosed by Ford in U.S. Pat. No. 5,392,659. The solids trapped between the side plates and the rear plate of the sampler and the surface of the belt are accelerated in a direction normal their motion on the belt and are thrown off the side of the belt into a chute of suitable geometry (not shown). With the objective of sweeping off the belt any particles remaining within the path of the sampler head, there is often an adjustable wiper or scraper or brush fixed to the trailing edge of the sampler; a wiper is usually made of a flexible material and may be adjustable. The use of a brush at the trailing edge of a cross-belt sampler is described in van der Merwe, in U.S. Pat. No. 5,115,688.
Note that the profile of the belt must be closely controlled with special idlers to ensure that is a section of a cylinder in section normal to the direction of travel of the belt (as it is shown in FIG. 1) or that the design of the sampler is such that it effectively exerts a pressure on the belt as it moves over the belt so that the edges of the sampler remain in good contact with the belt, preventing particles that should be part of the sample increment from flowing under the edges of the sampler and being lost. Long considers this problem, U.S. Pat. No. 5,767,421. The technical descriptions provided herein assume that such shaping and/or support of the belt has been accomplished. If such constraint of the belt has not been accomplished, a cross-belt sampler cannot function in a technically useful manner.
In a more complex design, the two parallel plates delimiting the sample are set at an angle to the motion of the sampler head. The angle may be chosen so that the vector direction of the velocity of the solids at the surface of the belt relative to the cross-belt sampler is parallel to the plates. If the speed of the cross-belt sampler at its extreme radius is equal to the speed of the belt, the angle is set to 45 degrees. If the speed of the cross-belt sampler at the belt surface is √{square root over (3)}vB, where vB is the speed of the belt, the angle is 60 degrees. A sampler with angled side plates is displayed in FIG. 2. The inclination of the plates permits the solids to enter the sampler with a velocity relative to the moving sampler that is directed approximately parallel to the side plates. To maintain close contact with the belt, the side plates must have a partially elliptical shape where they contact the belt. This design with angled side plates is disclosed in van der Merwe, U.S. Pat. No. 5,115,688.
It is also known (see ISO 13909-2-2001 Hard coal and coke—Mechanical sampling—Part 2—Coal—Sampling from moving streams) to have a sample cutter of a geometry as shown in FIG. 1 which rotates on an axis as indicated in FIG. 1 but which is carried on a moveable trolley above the belt, the trolley being put into motion parallel to the motion of the belt before the sampling device contacts the belt or the solids on the belt. The trolley moves at the same speed as the belt. Such a sampler is capable of collecting the material between its parallel side plates and the swath of the device on the belt is at right angles to the motion of the belt.
The three embodiments of the cross-belt sampler described above constitute the state of the art in cross-belt sampler design.
The International Standards (for example, Australian Standard AS 4264.1-1995 and ISO 13909-2-2001 Hard coal and coke—Mechanical sampling—Part 2—Coal—Sampling from moving streams) that describe sampling devices and the cross-belt sampler in particular consider the cross-belt sampler to be a somewhat flawed device and it is generally believed that cross-belt samplers take biased samples. It is also relevant that Australian Standard AS 4264.1-1995 requires that the absolute speed of the sampler head at the surface of the belt exceed 1.5 times the belt speed. The Standard states that cross-belt samplers are not to be used for commercial purposes except where the device can be shown to be free of a defined level of bias that is acceptable under the particular circumstances of sampling.
The reason for the possible bias in the sample taken by the cross-belt sampler has not previously been explained. For some materials and applications, the inherent bias of the cross-belt sampler design, while it continues to exist, is not large enough to discourage or preclude the use of the machine. Even a poorly designed sampler will appear to be unbiased if it is used to sample an essentially homogeneous material. Because of the general failure by sampling technologists to understand why the conventional designs of cross-belt sampler are biased, the tolerance of bias at a level deemed insignificant in a given circumstance is condoned. Much of the testing for significant bias of the devices has been carried out in a manner that precludes the application of robust statistical method to the test results.